Coated Nickel Hydroxide Powder for Positive Electrode Active Material of  Alkaline Secondary Battery, and Production Method Therefor

ABSTRACT

The pH of a suspension of a nickel hydroxide powder is kept at 8 to 11.5, and an aqueous cobalt salt solution and an aqueous alkali, solution are supplied to the suspension while the ratio of the supply rate ρ of the aqueous cobalt salt solution to the product of the supply width d of the aqueous cobalt salt solution in a direction orthogonal to the flow direction of the suspension and the flow velocity v of the suspension in a contact portion between the suspension and the aqueous cobalt salt solution, that is, ρ/(d×v) is controlled to be 3.8×10−4 mol/cm2 or less to coat the surface of nickel hydroxide particles with cobalt hydroxide. Then, the pH of a slurry of the cobalt-hydroxide-coated nickel hydroxide powder is adjusted to 12.5 or higher, and oxygen is supplied to the slurry so that the total amount of oxygen supplied per mole of cobalt in the coating is 30 l/mol or more to oxidize cobalt hydroxide.

TECHNICAL FIELD

The present invention relates to a nickel hydroxide powder for apositive electrode active material of alkaline secondary battery and amethod for producing such a nickel hydroxide powder. Particularly, thepresent invention relates to a coated nickel hydroxide powder coatedwith a cobalt compound to ensure conductivity between particles thereofand enhance the utilization ratio, lifetime characteristics, and outputcharacteristics of a battery, and a method for producing such a coatednickel hydroxide powder.

BACKGROUND ART

With recent development of portable devices, there has been a strongdemand for higher-capacity secondary batteries for use in such devices.For example, a nickel hydroxide powder for a positive electrode materialfor alkaline secondary battery has been improved by forming a solidsolution with cobalt to improve the utilization ratio of an alkalinesecondary battery at high temperatures or by forming a solid solutionwith zinc or magnesium to improve the lifetime characteristics of analkaline secondary battery.

Alkaline secondary batteries have come to be used as high-output powersources such as power sources for hybrid cars; therefore, there has beena strong demand not only for such improvement in utilization ratio athigh temperatures or lifetime characteristics but also for improvementin output characteristics. However, a nickel hydroxide powder for apositive electrode active material of alkaline secondary battery is anelectrical insulating material and poor in conductivity, which causes aproblem that an electrical current does not sufficiently run throughnickel hydroxide; therefore, the electrochemical availability of nickelhydroxide is low.

In order to solve such a problem, a cobalt compound such as cobalt oxideor cobalt hydroxide is added as a conductive material to ensureconductivity between nickel hydroxide particles. Such a cobalt compoundadded is dissolved in a high-concentration alkali metal hydroxidesolution used as an electrolyte in an alkaline secondary battery, and isoxidized and deposited as cobalt oxyhydroxide on the surface of nickelhydroxide particles during electrical charge so that electricalconductivity is developed and a conductive network is formed between thenickel hydroxide particles.

A positive electrode using a nickel hydroxide powder and such a cobaltcompound as an additive is generally produced by the steps of mixing anickel hydroxide powder, a cobalt compound powder, and a binder to forma paste, filling the pores of a three-dimensional metal porous body,such as a foamed metal (made of nickel metal), with the paste, andsubjecting the three-dimensional metal porous body to drying andpressing. However, the cobalt compound powder mixed together with thebinder is not necessarily well dispersed in the nickel hydroxide powder.Therefore, the positive electrode has a problem that its utilizationratio is significantly reduced under the conditions of use duringhigh-load electrical charge.

As a means for solving such a problem, a method has been proposed inwhich the surface of particles of a nickel hydroxide powder is coatedwith a cobalt compound. For example, Patent Literature 1 proposes anickel active material for storage battery mainly comprising nickelhydroxide particles having a β-type cobalt hydroxide thin layer formedthereon. Patent Literature 1 states that this nickel active material isobtained by depositing a nickel hydroxide powder from a nickel salt inan aqueous alkali solution, immersing the nickel hydroxide powder in anaqueous solution of cobalt sulfate or cobalt nitrate, and neutralizingthe aqueous solution with an aqueous alkali solution.

Further, Patent Literature 2 proposes a method for producing a nickelhydroxide powder coated with cobalt hydroxide, in which acobalt-containing aqueous solution and an ammonium ion supplier aresimultaneously, continuously, and quantitatively supplied to a nickelhydroxide powder-containing aqueous solution adjusted to pH 11 to 13with a caustic alkali.

Further, Patent Literature 3 proposes a method in which a cobaltion-containing aqueous solution is supplied to a suspension of a rawnickel hydroxide powder at a supply rate of 0.7 g/min or less in termsof cobalt per kilogram of the raw nickel hydroxide powder, and anammonium ion-containing aqueous solution is supplied to the suspensionto achieve a nickel ion concentration of 10 to 50 mg/l and a cobalt ionconcentration of 5 to 40 mg/l while the pH, temperature, and ammoniumion concentration of the suspension are kept at predetermined values.

CITATION LIST Patent Literatures

Patent Literature 1: JP 63-152866 A

Patent Literature 2: JP 7-133115 A

Patent Literature 3: JP 2000-149941 A

SUMMARY OF INVENTION Technical Problem

All the above methods described in Patent Literatures 1 to 3 areintended to ensure the dispersibility and uniformity of a conductivecobalt compound by previously coating the surface of particles of anickel hydroxide powder with cobalt hydroxide. However, suchconventional methods have a problem that a cobalt hydroxide coating isnon-uniformly formed on the surface of nickel hydroxide particles or ispeeled off in the process of paste preparation; therefore, it isdifficult to ensure the uniformity and adhesion properties of aconductive cobalt compound.

In view of the circumstances, it is an object of the present inventionto provide a method for producing a coated nickel hydroxide powdersuitable for a positive electrode active material of alkaline secondarybattery, in which a coating mainly containing cobalt oxyhydroxide or amixture of cobalt oxyhydroxide and cobalt hydroxide is formed on thesurface of particles of a nickel hydroxide powder in an aqueous solutionwhile the uniformity and adhesion properties of the coating are ensured.

Solution to Problem

In order to achieve the above object, the present inventors haveintensively studied a method for coating the surface of nickel hydroxideparticles with cobalt oxyhydroxide, and as a result have found that thestate of diffusion of an aqueous cobalt salt solution and an aqueousalkali solution in a suspension, obtained by dispersing a nickelhydroxide powder in water, during formation of a cobalt hydroxidecoating has a great effect on the uniformity and adhesion properties ofthe cobalt hydroxide coating. Further, the present inventors have alsofound that the adhesion properties of the coating can be improved byoxidizing the cobalt hydroxide coating to cobalt oxyhydroxide underoptimum conditions. These findings have led to the completion of thepresent invention.

More specifically, the present invention is directed to a productionmethod for a coated nickel hydroxide powder for a positive electrodeactive material of alkaline secondary battery comprising: a coating stepin which an aqueous cobalt salt solution and an aqueous alkali solutionare supplied to a stirred suspension obtained by dispersing a nickelhydroxide powder in water to form, on a surface of nickel hydroxideparticles, a coating mainly containing cobalt hydroxide crystallized outby neutralization; and an oxidation step in which oxygen is supplied toa stirred slurry of the nickel hydroxide particles having the coatingformed thereon to oxidize cobalt hydroxide in the coating, wherein

in the coating step, a ratio of a supply rate (ρ) of the aqueous cobaltsalt solution to a product of a supply width (d) of the aqueous cobaltsalt solution in a direction orthogonal to a flow direction of thesuspension and a flow velocity (v) of the suspension in a contactportion between the suspension and the aqueous cobalt salt solution,that is, ρ/(d×v) is controlled to be 3.5×10⁻¹ mol/cm² or less while a pHof the suspension mixed with the aqueous cobalt salt solution and theaqueous alkali solution as measured at 25° C. is kept at 8 to 11.5, and

in the oxidation step, a pH of the slurry is adjusted to 12.5 or higheras measured at 25° C., and a total amount of oxygen supplied by blowinginto the slurry per mole of cobalt in the coating is 30 l/mol or more.

The present invention is also directed to a coated nickel hydroxidepowder for a positive electrode active material of alkaline secondarybattery, comprising nickel hydroxide powder particles having, on asurface thereof, a coating made of a cobalt compound, mainly containingcobalt oxyhydroxide or a mixture of cobalt oxyhydroxide and cobalthydroxide, wherein a valence of cobalt in the coating is 2.5 or more.Further, when 20 g of the coated nickel hydroxide powder for a positiveelectrode active material of alkaline secondary battery according to thepresent invention is shaken in a closed container for 1 hour, an amountof the coating peeled off is 20 mass % or less of a total amount of thecoating.

Advantageous Effects of Invention

According to the present invention, the surface of particles of a nickelhydroxide powder can be coated with cobalt hydroxide in a suspensionwhile the uniformity and adhesion properties of a coating mainlycontaining cobalt hydroxide are stably ensured. Further, the adhesionproperties of the coating can be further improved by oxidizing cobalthydroxide that coats the surface of the particles of the nickelhydroxide powder. Therefore, the coated nickel hydroxide powderaccording to the present invention has a uniform coating, mainlycontaining cobalt oxyhydroxide or a mixture of cobalt oxyhydroxide andcobalt hydroxide, formed on the surface of particles thereof and canprevent its coating from being peeled off in the process of preparing apaste by mixing with a binder or the like, and is therefore excellentfor a positive electrode active material of alkaline secondary battery.

Further, the coated nickel hydroxide powder according to the presentinvention not only can prevent peeling-off of the coating during pastepreparation but also has high conductivity, and is thereforeparticularly suitable as a positive electrode active material for use inan alkaline secondary battery used as a power source for electric car orhybrid car required to have high-output characteristics. Further, thecoated nickel hydroxide powder according to the present invention isimproved in utilization ratio due to its improved conductivity, and istherefore extremely effective as a positive electrode active materialfor use in an alkaline secondary battery used as a power source forportable electronic device required to have a high capacity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A scanning electron micrograph of a cobalt-oxyhydroxide-coatednickel hydroxide powder prepared in Example 1.

FIG. 2 A scanning electron micrograph of a cobalt-oxyhydroxide-coatednickel hydroxide powder prepared in Comparative Example 1.

DESCRIPTION OF EMBODIMENTS

In a general method for producing a coated nickel hydroxide powder for apositive electrode active material of alkaline secondary battery, anaqueous cobalt salt solution and an aqueous alkali solution are added toa stirred suspension obtained by dispersing a nickel hydroxide powder inwater so that the surface of the nickel hydroxide powder is coated withcobalt hydroxide by a crystallization reaction caused by neutralization.In this method, cobalt is present in its ionic state in a region of thesuspension where a pH value is low, but deposition of cobalt hydroxidegradually starts as the pH value increases. At this time, when nickelhydroxide particles are present near cobalt hydroxide, the cobalthydroxide is deposited on the surface of the nickel hydroxide particlesthat are energetically more stable.

The present inventors have closely studied such a deposition process ofcobalt hydroxide, and as a result have found that when the concentrationof cobalt ions rapidly increases and exceeds its criticalsupersaturation in the pH range of the suspension in which cobalt ionsare present, cobalt hydroxide is deposited alone in water even whennickel hydroxide particles are not present near the cobalt hydroxide.However, when the pH value of the suspension is low, more specificallywhen the pH is less than 8, the deposition rate of cobalt hydroxide islow; therefore, cobalt hydroxide is not deposited alone even when theconcentration of cobalt ions exceeds it critical supersaturation.

On the other hand, when the pH value of the suspension in which cobaltions are present is 8 or higher, the critical supersaturation of theconcentration of cobalt ions is reduced. Therefore, the concentration ofcobalt ions easily exceeds its critical supersaturation when itincreases. As a result, cobalt hydroxide is deposited alone withoutadhering to the surface of nickel hydroxide particles. It has been foundthat such cobalt hydroxide deposited alone adheres to the surface ofnickel hydroxide particles when a nickel hydroxide slurry is filtered,but the cobalt hydroxide sparsely adheres to the surface of nickelhydroxide particles and is therefore poor in uniformity, and inaddition, the cobalt hydroxide is very poor in adhesion propertiesbecause adhesion between the cobalt hydroxide and the nickel hydroxideparticles is achieved simply by filtration.

Based on the results of the study of such a deposition process of cobalthydroxide, in order to uniformly form cobalt hydroxide having highadhesion properties on the surface of particles of a nickel hydroxidepowder in a coating step of a method according to the present invention,the pH (as measured at 25° C.) of a suspension of the nickel hydroxideparticles is controlled to be 8 or higher, and the concentration ofcobalt ions in the suspension in such a pH range is kept at or below aconcentration at which cobalt hydroxide is not deposited alone.According to such a method, cobalt hydroxide is deposited on the surfaceof nickel hydroxide particles in accordance with the surface structureof nickel hydroxide; therefore, a coating having extremely high adhesionproperties is uniformly formed on the surface of the particles.

In order to reliably deposit such cobalt hydroxide excellent inuniformity and adhesion properties, it is important to avoid thecreation of a region where the concentration of cobalt ions is high.This is achieved by reducing the ratio of the supply rate of the aqueouscobalt salt solution to the amount of the suspension flowing into aportion where the aqueous cobalt salt solution is supplied to and mixedwith the suspension. That is, it is necessary to prevent the appearanceof a region where the concentration of cobalt ions is extremely high inthe suspension. This is achieved by reducing the supply rate of theaqueous cobalt salt solution to sufficiently reduce the concentration ofa cobalt salt even when the amount of the suspension mixed with theaqueous cobalt salt solution is small, or by increasing the amount ofthe suspension mixed with the aqueous cobalt salt solution to diffusethe aqueous cobalt salt solution supplied to the suspension as quicklyas possible for dilution.

The amount of the suspension mixed with the aqueous cobalt salt solutionmay be considered as the amount of the suspension flowing into a portionwhere the aqueous cobalt salt solution supplied comes into contact withthe surface of the suspension and is mixed with the suspension. Themixing is initially performed in an extremely short period of time;therefore, when the flow velocity of the suspension is sufficientlyhigh, the amount of suspension mixed with the aqueous cobalt saltsolution can be considered as the surface of the suspension that comesinto contact with the aqueous cobalt salt solution per unit time. Thatis, the amount of the suspension mixed with the aqueous cobalt saltsolution can be considered as the product of the supply width (d) of theaqueous cobalt salt solution in a direction orthogonal to the flowdirection of the suspension and the surface flow velocity (v) of thesuspension in a contact portion between the surface of the suspensionand the aqueous cobalt salt solution. It is to be noted that when thecontact portion between the surface of the suspension and the aqueouscobalt salt solution is circular, the supply width (d) of the aqueouscobalt salt solution in a direction orthogonal to the flow direction ofthe suspension is equal to the diameter of the circular contact portion.Further, when it is difficult to actually measure the surface flowvelocity of the suspension, the surface flow velocity of the suspensioncan be easily determined by simulation.

In the present invention, the ratio of the supply rate ρ (mol/sec) ofthe aqueous cobalt salt solution to the product of the supply width d(cm) of the aqueous cobalt salt solution in a direction orthogonal tothe flow direction of the suspension and the flow velocity v (cm/sec) ofthe suspension in a contact portion between the surface of thesuspension and the aqueous cobalt salt solution, that is, ρ/(d×v) needsto be small. More specifically, the ratio needs to be 3.5×10⁻⁴ mol/cm²or less, and is preferably 2.0×10⁻⁴ mol/cm₂ or less. If the ratioρ/(d×v) exceeds 3.5×10⁻⁴ mol/cm², a region where the concentration ofcobalt ions is high appears so that cobalt hydroxide is deposited alone.It is to be noted that the lower limit of the ratio ρ/(d×v) is notparticularly limited, but is preferably 0.01×10⁻⁴ mol/cm² or morebecause a reduction in the supply rate (ρ) reduces productivity.

Here, when the opening size of a supply port for the aqueous cobalt saltsolution is small enough to supply the aqueous cobalt salt solution as astable liquid flow whose cross-section is substantially the same in sizeas the opening of the supply port, the area of the contact portion, thatis, the area of a portion where the aqueous cobalt salt solutionsupplied from the supply port toward the surface of the suspension firstcomes into contact with the surface of the suspension coincides with theprojected area of the supply port onto the surface of the suspension.That is, when the opening size of the supply port for the aqueous cobaltsalt solution is small as described above, the area of the contactportion between the surface of the suspension and the aqueous cobaltsalt solution may be regarded as the projected area of the supply portonto the surface of the suspension. On the other hand, when the openingsize of the supply port for the aqueous cobalt salt solution is largerthan that described above, the flow velocity of the aqueous cobalt saltsolution discharged from the supply port is generally low, which makesit impossible to uniformly supply the aqueous cobalt salt solution fromthe supply port and makes it difficult to control the area of thecontact portion between the surface of the suspension and the aqueouscobalt salt solution.

Therefore, in order to supply the aqueous cobalt salt solution as astable liquid flow from the supply port, the area of opening of thesupply port is preferably 0.01 to 1.0 cm². If the area of opening of thesupply port is less than 0.01 cm², there is a case where the supply rateof the aqueous cobalt salt solution is low; therefore, sufficientproductivity cannot be achieved. On the other hand, if thecross-sectional area of the supply port exceeds 1.0 cm², there is a casewhere the aqueous cobalt salt solution is not sufficiently diffusedbecause it is difficult to uniformly supply the aqueous cobalt saltsolution from the supply port; therefore, the amount of the aqueouscobalt salt solution supplied varies even when the contact portionbetween the surface of the suspension and the aqueous cobalt saltsolution is within a region defined by the projected area of the supplyport onto the surface of the suspension so that the aqueous cobalt saltsolution is likely to be intensively supplied to a particular portion.

It is to be noted that when the aqueous cobalt salt solution is suppliedby spraying it from the supply port onto the surface of the suspensionwith a spray nozzle or the like, the area of contact between the aqueouscobalt salt solution and the surface of the suspension can be regardedas the area of a region where the aqueous cobalt salt solution issprayed onto the surface of the suspension. Alternatively, two or moresupply ports may be provided to increase the total amount of the aqueouscobalt salt solution supplied to increase productivity as long as, asdescribed above, the aqueous cobalt salt solution can be uniformlysupplied from the supply ports to the surface of the suspension. Thenumber of supply ports is not particularly limited, and may bedetermined in consideration of the supply rate of the aqueous cobaltsalt solution supplied from each of the supply port or the product ofthe supply width of the aqueous cobalt salt solution and the flowvelocity of the suspension.

Further, also when the pH value of the suspension rapidly increases in aportion where the aqueous cobalt salt solution is supplied, theconcentration at which cobalt hydroxide is not deposited alone in such ahigh pH range is reduced so that cobalt hydroxide is easily depositedalone. As a result, cobalt hydroxide is started to be deposited aloneeven when nickel hydroxide particles are not present near the cobalthydroxide; therefore cobalt hydroxide poor in adhesion properties anduniformity is likely to adhere to the surface of nickel hydroxideparticles. In order to prevent this, it is preferred that the aqueousalkali solution supplied simultaneously with the aqueous cobalt saltsolution is also diffused at a sufficiently high speed to inhibit thecreation of a high pH region due to a rapid increase in theconcentration of the aqueous alkali solution.

For example, if the supply rate of a cobalt salt to the surface of thesuspension per unit area exceeds 0.01 mol/cm²·min even when the flowvelocity of the suspension is sufficiently high, a reaction occurs dueto contact between a high pH region and the aqueous cobalt salt solutionbefore the aqueous alkali solution is sufficiently diffused in thesuspension when the distance between the supply position of the aqueouscobalt salt solution added and the supply position of the aqueous alkalisolution added is short. In this case, there is a high possibility thatcobalt hydroxide poor in adhesion properties and uniformity isdeposited.

In order to avoid this, the ratio of the distance (D) (cm) of separationbetween the supply position of the aqueous cobalt salt solution and thesupply position of the aqueous alkali solution to the above-describedratio of the supply rate ρ of the aqueous cobalt salt solution to theproduct of the supply width d of the aqueous cobalt salt solution andthe flow velocity v of the suspension {ρ/(d×v)}, that is, D/{ρ/(d×v)} ispreferably 0.5×10⁵ cm³/mol or more, more preferably 1.0×10⁵ cm³/mol ormore. It is to be noted that the upper limit of the ratio D/{ρ/(d×v)} isnot particularly limited, but is limited by the supply rate (ρ) of theaqueous cobalt salt solution or the size of a reactor, and is thereforepreferably about 100×10⁵ cm³/mol.

Here, nickel hydroxide used as a core material to be coated with cobalthydroxide may be one known for a positive electrode active material ofalkaline secondary battery, but is particularly preferably a nickelhydroxide represented by the following general formula:Ni_(1-x-y)Co_(x)M_(y)(OH)₂ (wherein x is 0.005 to 0.05, y is 0.005 to0.05, M is one or more of Ca, Mg, and Zn).

If x that represents a cobalt content in the above general formula isless than 0.005, the effect of improving charge efficiency achieved byadding cobalt cannot be obtained. On the other hand, if x exceeds 0.05,battery performance is degraded due to a reduction in discharge voltage.If y that represents the amount of M contained as an additive element inthe above formula is less than 0.005, the effect of reducing a change inthe volume of nickel hydroxide during discharge and charge achieved byadding the element M cannot be obtained. On the other hand, if y exceeds0.05, the effect of reducing a change in the volume of nickel hydroxidecan be obtained, but beyond that, a reduction in battery capacity iscaused so that battery performance is undesirably degraded.

Hereinbelow, a coating process in the method for producing a coatednickel hydroxide powder according to the present invention will be morespecifically described. It is to be noted that the production methodaccording to the present invention can achieve improvement inproductivity when performed in a continuous manner, but is preferablyperformed in a batch manner to form a uniform coating on nickelhydroxide particles. Therefore, the following production method will bedescribed with reference to a case of a batch manner.

First, a suspension of a nickel hydroxide powder, an aqueous solution ofa cobalt salt, and an aqueous solution of an alkali are prepared. Thenickel hydroxide powder used as a core material preferably has anaverage particle size of 6 to 12 μm so that a battery using a resultingcoated-nickel hydroxide powder as a positive electrode material canachieve excellent battery characteristics. Further, the concentration ofthe nickel hydroxide in the suspension is preferably 400 to 1200 g/l. Ifthe concentration thereof is less than 400 g/l, there is a case wherecobalt hydroxide is deposited alone in the suspension due to the lack ofsurface active sites of nickel hydroxide particles where deposition ofcobalt hydroxide occurs. On the other hand, the concentration of thenickel hydroxide exceeds 1200 g/l, there is a case where the suspensioncannot be sufficiently stirred due to an increase in viscosity so that acobalt hydroxide coating is non-uniformly formed.

The cobalt salt is not particularly limited as long as the cobalt saltis a water-soluble cobalt compound from which cobalt hydroxide isgenerated by pH control. More specifically, the cobalt salt ispreferably cobalt sulfate or cobalt chloride, more preferably cobaltsulfate not contaminated with halogens. The alkali is not particularlylimited, but is preferably water-soluble sodium hydroxide or potassiumhydroxide, and is particularly preferably sodium hydroxide from theviewpoint of costs.

The suspension of the nickel hydroxide powder is preferably prepared bydispersing nickel hydroxide powder in pure water or the like to preventimpurity incorporation. The aqueous cobalt salt solution or the aqueousalkali solution is also preferably prepared by dissolving a cobalt saltor an alkali in pure water, respectively. It is to be noted that theconcentrations of the aqueous cobalt salt solution and the aqueousalkali solution are not particularly limited as long as redepositiondoes not occur in tubes or the like of an apparatus used and a problemdoes not occur even when the concentration of nickel hydroxide in thesuspension varies, and an aqueous cobalt salt solution and an aqueousalkali solution having predetermined concentrations that depend on, forexample, the concentration of the suspension can be used.

When the production method is performed in a batch manner, the aqueouscobalt salt solution and the aqueous alkali solution for forming acoating are continuously supplied to a reactor containing the stirredsuspension of the nickel hydroxide powder used as a core material. As aresult, the surface of nickel hydroxide particles is coated with cobalthydroxide crystallized out by neutralization so that acobalt-hydroxide-coated nickel hydroxide powder is produced. The reactorused in the batch-wise production method is not particularly limited,but preferably has a stirring device and a liquid temperature-regulatingsystem to form a uniform coating on the surface of particles of thenickel hydroxide powder.

The aqueous cobalt salt solution and the aqueous alkali solution need tobe supplied individually, but may be supplied at the same time as longas they are supplied individually. The aqueous cobalt salt solution andthe aqueous alkali solution may be supplied together with a part of thesuspension to the residual suspension contained in the reactor. However,when all these liquids are previously mixed and supplied as a mixedliquid to the reactor, there is a case where a reaction occurs in themixed liquid so that cobalt hydroxide is deposited alone. Further, whenthe aqueous cobalt salt solution and the aqueous alkali solution are notsupplied to the suspension individually, there is a case where theamount of a cobalt hydroxide coating formed on the surface of nickelhydroxide particles is not uniform among the particles.

The pH of the suspension at the time when the aqueous cobalt saltsolution and the aqueous alkali solution supplied are mixed until anequilibrium state is achieved is kept in the range of 8 to 11.5 asmeasured at 25° C., and is preferably kept in the range of 9.5 to 10.5as measured at 25° C. If the pH value of the suspension is less than 8,the deposition rate of cobalt hydroxide is too low; therefore,productivity is reduced. On the other hand, if the pH value of thesuspension exceeds 11.5, there is a case where generated cobalthydroxide is likely to gelate; therefore, it is difficult to form anexcellent cobalt hydroxide coating.

Further, the pH of the suspension is preferably kept at a certain valuein the range of 8 to 11.5 as measured at 25° C. and controlled so thatits fluctuation range is within ±0.2. If the fluctuation range of the pHexceeds the above limit, there is a fear that the amount of a cobalthydroxide coating varies. It is preferred that the pH of the suspensionis continuously measured with, for example, a pH controller using aglass electrode method, and the flow rate of the aqueous alkali solutionis continuously feedback-controlled with the pH controller so that thepH is kept constant within the above fluctuation range.

The temperature of the suspension is preferably in the range of 30 to60° C. before and after the aqueous cobalt salt solution and the aqueousalkali solution are added. If the temperature of the suspension is lessthan 30° C., cobalt hydroxide is slowly deposited due to a low reactionrate. On the other hand, if the temperature of the suspension exceeds60° C., cobalt hydroxide is likely to be non-uniformly deposited on thesurface of nickel hydroxide particles due to too high a reaction rate.Further, the temperature of the suspension is preferably kept at acertain value within the above temperature range and controlled so thatits fluctuation range is ±1° C. If the fluctuation range of thetemperature of the suspension exceeds the above limit, there is a fearthat the concentration of impurities in deposited cobalt hydroxidevaries so that a battery using a resulting coated nickel hydroxidepowder does not have stable characteristics.

In the above-described coating step, a coating that is excellent inuniformity and adhesion properties and mainly contains cobalt hydroxidecan be formed on the surface of particles of a nickel hydroxide powder.According to the production method of the present invention, cobalthydroxide constituting the coating is oxidized to cobalt oxyhydroxide ina next oxidation step, which makes it possible to further improve theadhesion properties of the coating and the conductivity of the coatednickel hydroxide powder.

More specifically, the production method for a coated nickel hydroxidepowder according to the present invention includes an oxidation step inwhich oxygen is supplied to a stirred slurry of the nickel hydroxidepowder having a cobalt hydroxide coating formed on the surface ofparticles thereof in the coating step to oxidize cobalt hydroxideconstituting the coating to cobalt oxyhydroxide according to thefollowing reaction formula represented by a chemical formula 1.

Co(OH)₂+¼O₂→CoOOH+½H₂O  [Chemical Formula 1]

Cobalt oxyhydroxide can be obtained also by oxidizing cobalt hydroxideusing an oxidizer such as sodium hypochlorite or persulfate. However,when the nickel hydroxide powder coated with cobalt hydroxide isoxidized with such an oxidizer, part of the nickel hydroxide powder as acore material is also oxidized so that the adhesion properties of thecobalt hydroxide coating is unstable due to generation of relativelyunstable nickel oxyhydroxide. Further, the use of an expensive oxidizersuch as sodium hypochlorite or persulfate is disadvantageous also interms of cost in consideration of industrial productivity.

In the oxidation step, the presence of OH ions in the slurry promotesthe oxidation reaction, which makes it possible to oxidize cobalthydroxide to cobalt oxyhydroxide without using an expensive oxidizer.Therefore, the concentration of OH ions in the slurry, more specificallythe pH of the slurry needs to be kept at 12.5 or higher as measured at25° C. in the oxidation step. By keeping the pH of the slurry at 12.5 orhigher, the oxidation reaction is promoted; therefore, cobalt hydroxidecan be oxidized to cobalt oxyhydroxide by supplying oxygen into theslurry. If the pH of the slurry is less than 12.5, the oxidationreaction is not sufficiently promoted and requires a long reaction time,which reduces industrial productivity. If the reaction is terminated ina short period of time, cobalt hydroxide is insufficiently oxidized tocobalt oxyhydroxide so that the adhesion properties of the coating islow and the conductivity of the coated nickel hydroxide powder is alsopoor.

On the other hand, the pH of the slurry in the oxidation step exceeding13.5 only increases the cost of an alkali used. This is because even ifthe pH of the slurry in the oxidation step exceeds 13.5, the effect ofpromoting oxidation is not higher than that obtained when the pH of theslurry is 13.5 or less. Therefore, the pH of the slurry in the oxidationstep is preferably kept at 12.5 or higher but 13.5 or less as measuredat 25° C., more preferably 12.5 or higher but 13.0 or less as measuredat 25° C. Further, if the concentration of OH ions frequently varies,there are variations in oxidation to cobalt oxyhydroxide, whichadversely affects the battery characteristics of a battery using theresulting coated nickel hydroxide powder as a positive electrodematerial. For this reason, the fluctuation range of the pH of the slurryin the oxidation step is preferably ±0.2, more preferably ±0.1.

In the oxidation step, the total amount of oxygen supplied by blowinginto the slurry per mole of cobalt in the coating is 30 l/mol or more.As can be seen from the above chemical formula 1, the amount of oxygenthat needs to be supplied to complete the reaction is ¼ of the molarquantity of cobalt contained in the cobalt hydroxide coating to beoxidized, which corresponds to 5.6 l in terms of the amount of oxygenunder standard conditions. Oxygen gas or an oxygen-containing gassupplied in the oxidation step is dispersed as bubbles in the slurry,and is partially dissolved in the solvent of the slurry. However, allthe oxygen gas or oxygen-containing gas does not contribute to theoxidation reaction of cobalt hydroxide, and most of the oxygen gas oroxygen-containing gas is directly discharged to the outside of areaction system. Therefore, it is necessary to supply oxygen in anamount of 6 to 20 times the amount of oxygen that needs to be suppliedfor the reaction represented by the above chemical formula.

If the total amount of oxygen supplied is less than 30 l/mol, part ofcobalt hydroxide insufficiently subjected to the oxidation reactionremains so that the adhesion properties of the coating is reduced. Onthe other hand, even if the total amount of oxygen supplied exceeds 110l/mol, oxygen is wastefully supplied as this cause oxygen supply evenafter the completion of the oxidation of cobalt hydroxide in thecoating, and in addition, nickel hydroxide as a core material may beoxidized. For this reason, the total amount of oxygen supplied per moleof cobalt in the cobalt hydroxide coating is preferably 30 l/mol or morebut 110 l/mol or less.

The supply of oxygen is preferably performed so that the amount ofoxygen supplied per unit time is 0.2 to 0.45 l/min·mol. If the amount ofoxygen supplied per unit time is less than 0.2 l/min·mol, the oxidationreaction of cobalt hydroxide to cobalt oxyhydroxide is unnecessarilyslow, which is impractical due to reduction in industrial productivity.On the other hand, if the amount of oxygen supplied per unit timeexceeds 0.45 l/min·mol, there is a case where the supply rate of oxygenis much higher than the oxidation rate of cobalt hydroxide; therefore,the oxidation reaction of cobalt hydroxide is inefficient so that cobalthydroxide is not sufficiently oxidized. Further, there is also a casewhere high adhesion properties cannot be achieved due to non-uniformoxidation state of cobalt hydroxide. For this reason, the amount ofoxygen supplied per unit time is set to 0.2 to 0.45 l/min·mol so thatcobalt hydroxide can be properly and efficiently oxidized.

Further, the supply of oxygen is preferably performed for 2.5 to 4.0hours. If the supply time of oxygen is less than 2.5 hours, there is acase where the reaction does not proceed to the extent that cobalthydroxide is sufficiently oxidized irrespective of the amount of oxygensupplied per unit time. On the other hand, even if the supply time ofoxygen exceeds 4.0 hours, oxygen is only wasted, and in addition, thereis a case where the oxidation reaction excessively proceeds; therefore,nickel hydroxide as a core material is also oxidized so that theadhesion properties of the coating is reduced. For this reason, thesupply time of oxygen is set to 2.5 to 4.0 hours so that cobalthydroxide can be efficiently and sufficiently oxidized.

In the oxidation step, the temperature of the slurry during theoxidation reaction is preferably kept in the range of 40 to 60° C. Ifthe temperature of the slurry is less than 40° C., there is a case wherethe reaction rate of the oxidation reaction is reduced so that theoxidation reaction of cobalt hydroxide to cobalt oxyhydroxide does notefficiently proceed. On the other hand, if the temperature of the slurryexceeds 60° C., the oxidation reaction excessively proceeds; therefore,unstable nickel oxyhydroxide is generated due to the oxidation of nickelhydroxide as a core material so that the adhesion properties of thecoating is reduced.

The supply of oxygen to the slurry is not particularly limited to oneusing pure oxygen gas as long as the amount of oxygen contained in a gassupplied by blowing into the slurry satisfies the above-describedrequirements. Therefore, an oxygen-containing gas as well as oxygen gasmay be used. Examples of the oxygen-containing gas include air, a mixedgas obtained by mixing oxygen and air in any ratio, and a mixed gas ofoxygen and an inert gas. In consideration of handleability and cost, airis preferably used. When such a gas other than oxygen is used, themethod according to the present invention can be applied by convertingthe above-described total amount of oxygen supplied and the amount ofoxygen supplied per unit time to the total amount of the gas suppliedand the amount of the gas supplied per unit time, respectively, based onthe amount of oxygen contained in the gas supplied.

The slurry used in the oxidation step can be obtained by adjusting thepH of the suspension after the completion of the coating step.Alternatively, the slurry may be prepared by subjecting the suspensionafter the completion of the coating step to solid-liquid separation tocollect the powder and again dispersing the collected powder in water.In this case, the collected powder is preferably in a wet state whenagain dispersed. This is because when the collected powder is dried, thecoating formed on the surface of the particles in the coating step isexcessively oxidized so that the adhesion properties of the coating isreduced. It is to be noted that the concentration of the slurry in theoxidation step may be the same as that of the suspension in the coatingstep.

A device used in the oxidation step is not particularly limited as longas the device has a system capable of adjusting the temperature of theslurry and blowing a gas into the slurry while stirring the slurry. Forexample, a commonly-used reactor equipped with the above system issuitably used. Further, in order to stabilize the amount of blowingoxygen, the reactor is preferably one whose inside is shielded fromoutside air so that an atmosphere in the reactor can be controlled.

A coated nickel hydroxide powder for a positive electrode activematerial of alkaline secondary battery according to the presentinvention is obtained by the production method for a coated nickelhydroxide powder according to the present invention comprising thecoating step and the oxidation step. The coated nickel hydroxide powderaccording to the present invention comprises nickel hydroxide powderparticles having, on the surface thereof, a coating made of a cobaltcompound mainly containing cobalt oxyhydroxide or a mixture of cobaltoxyhydroxide and cobalt hydroxide.

The valence of cobalt in the cobalt compound coating mainly containingcobalt oxyhydroxide or a mixture of cobalt oxyhydroxide and cobalthydroxide of the coated nickel hydroxide powder according to the presentinvention is 2.5 or more, preferably 2.7 or more. When the valence ofcobalt in the coating is 2.5 or more, a necessary amount of cobaltoxyhydroxide is generated in the coating so that excellent adhesionproperties and conductivity are achieved. It is to be noted that theupper limit of valence of cobalt is usually 3.

Further, the amount of cobalt contained in the coating is preferably inthe range of 3 to 7 mass % with respect to the total mass of the nickelhydroxide particles as a core material and the coating, that is, thetotal mass of the coated nickel hydroxide powder. If the amount ofcobalt in the coating is less than 3 mass %, the effect of coating thesurface of the cobalt hydroxide particles is not sufficiently obtaineddue to the shortage of the amount of the cobalt compound constitutingthe coating. On the other hand, if the amount of cobalt in the coatingexceeds 7 mass %, the amount of the cobalt compound constituting thecoating is increased, but the coating effect is not further enhanced.Further, the nickel hydroxide particles as a core material arepreferably uniformly coated with the coating. The nickel hydroxideparticles may be coated with small islands shape of the coating as longas the nickel hydroxide particles are uniformly coated, but are morepreferably coated with a layer of the coating, and even more preferablyfully coated with the coating.

Further, the cobalt compound coating mainly contains cobalt oxyhydroxideor a mixture of cobalt oxyhydroxide and cobalt hydroxide. The amount ofcobalt contained in the coating is preferably 90 mass % or more, morepreferably 95 mass % or more with respect to the total mass of metalelements contained in the coating. An additive element, such as Ca, Mg,or Zn, can be added to the cobalt compound coating to improve thebattery characteristics of a battery having a cathode using the coatednickel hydroxide powder. However, if the amount of cobalt contained inthe coating is less than 90 mass %, there is a case where conductivitycannot be sufficiently improved by the coating.

The coated nickel hydroxide powder according to the present invention isexcellent in the uniformity and adhesion properties of the cobaltcompound coating formed on the surface of the nickel hydroxideparticles. Such excellent uniformity and adhesion properties of thecoating makes it possible, when 20 g of the coated nickel hydroxidepowder contained in a closed container is shaken for 1 hour, to limitthe amount of the coating peeled off to 20 mass % or less, preferably 19mass % or less of the total mass of the coating. If the amount of thecoating peeled off exceeds 20 mass %, there is a case where the cobaltcompound coating is peeled off during preparation of a paste so that theviscosity of the paste is unstable. In addition, the conductive cobaltcompound is non-uniformly present in a positive electrode of a battery;therefore, a conductive network between the nickel hydroxide particlesis not sufficiently formed, which results in poor batterycharacteristics such as positive electrode utilization efficiency.

The amount of the coating peeled off is measured in the followingmanner. First, a plastic circular cylindrical container is preparedwhich has such a capacity that the coated nickel hydroxide powdercontained therein can be sufficiently shaken. Then, the coated nickelhydroxide powder is placed in the container, and the container istightly closed. Then, the container is shaken by performingreciprocating movement in a direction parallel to the central axis ofthe container, rotation around the central axis of the container, andoscillation around the central point of the container in combination.Here, the closed container is preferably shaken at a reciprocatingstroke of 50 to 250 mm and a frequency of 30 to 60 times/min. Morespecifically, for example, a tightly-closed polyethylene jar having acapacity of 50 ml and containing the powder may be shaken with a TURBULAshaker mixer (container capacity: 2 L, e.g., TURBULA Type T2Cmanufactured by Willy A. Bachofen (WAB)). A device used for the shakingof the closed container is not limited to the device exemplified above,and may be any device capable of shaking the container in the samemanner. After the completion of the shaking, 10 g of the coated nickelhydroxide powder is mixed with 200 ml of pure water with stirring, andis then allowed to stand to separate the coating peeled off as asupernatant. Then, the settled coated nickel hydroxide powder iscollected and dried. The amount of the coating peeled off can bedetermined by comparison between the cobalt content of the collectedcoated nickel hydroxide powder and the cobalt content of the coatednickel hydroxide powder before shaking.

As described above, when the amount of the cobalt compound coatingpeeled off is limited to 20 mass % or less of the total mass of thecoating, the coating is not peeled off when the coated nickel hydroxidepowder is mixed with a binder or the like in the process of preparing apositive electrode paste for alkaline secondary battery, or even whenthe coating is peeled off, the ratio of the amount of the coating peeledoff to the total amount of the coating is low; therefore, conductivitybetween the nickel hydroxide particles in a positive electrode issufficiently ensured. For this reason, the coated nickel hydroxidepowder according to the present invention is extremely excellent for apositive electrode active material of alkaline secondary battery.

EXAMPLES Example 1

Six kilograms of a spherical nickel hydroxide powder having an averageparticle size of 8 μm was placed in a reactor having a diameter of 25 cmand a depth of 30 cm, and water was added to the reactor so that a totalvolume was 10 liters. Then, the nickel hydroxide powder was dispersed inthe water by stirring using a propeller stirrer at a rotation speed of500 rpm to prepare a suspension of the nickel hydroxide powder.

The suspension was kept stirred, and when the surface flow velocity ofthe suspension at a portion where a 1.6 mol/l aqueous cobalt sulfatesolution was to be added as an aqueous cobalt salt solution reached asteady state of 15.8 cm/sec, 2.017 liters of the aqueous cobalt sulfatesolution was added in 2 hours at a supply rate of 16.8 ml/sec using aroller pump from one supply port having a diameter of 2 mm. At the sametime, a 24 mass % aqueous sodium hydroxide solution was added from onesupply port to a portion where the surface flow rate of the suspensionwas the same as described above under control using a roller pumpinterfacing with a pH controller so that the pH of the suspension was10.2±0.2 as measured at 25° C. The supply port for the aqueous sodiumhydroxide solution was provided 15 cm away from the supply port for theaqueous cobalt sulfate solution in a horizontal direction, and had thesame diameter as the supply port for the aqueous cobalt sulfatesolution.

At this time, the ratio of the supply rate ρ (mol/sec) of the aqueouscobalt salt solution to the product of the supply width d (cm) of theaqueous cobalt salt solution in a direction orthogonal to the flowdirection of the suspension and the flow velocity v (cm/sec) of thesuspension in a contact portion between the surface of the suspensionand the aqueous cobalt salt solution supplied thereto, that is, ρ/(d×v)was 1.42×10⁻⁴ mol/cm². Further, the ratio of the distance D (cm) betweenthe supply position of the aqueous cobalt salt solution and the supplyposition of the aqueous alkali solution to the ratio of the supply rateρ of the aqueous cobalt salt solution to the product of the supply widthd of the aqueous cobalt salt solution and the flow velocity v of thesuspension, that is, D/{ρ/(d×v)} was 1.06×10⁵ cm³/mol. It is to be notedthat the temperature of the suspension during reaction was controlled tobe 50° C.

As a result of the coating step described above, the total amount ofcobalt sulfate supplied to the suspension was deposited as cobalthydroxide on the surface of particles of the nickel hydroxide powder sothat a cobalt-hydroxide-coated nickel hydroxide powder whose particleshad a cobalt hydroxide coating on the surface thereof was obtained.

After the completion of the coating step, sodium hydroxide was furtheradded to the stirred suspension to increase the pH of the suspension to12.8. In this way, a slurry to be used in the oxidation step wasobtained. Then, air was blown into the slurry at a flow rate of 3.5l/min for 4 hours to oxidize cobalt hydroxide deposited on the surfaceof the nickel hydroxide particles in the coating step to cobaltoxyhydroxide. At this time, the total amount of oxygen supplied per moleof cobalt in the coating was 52.0 l/mol, and the amount of oxygensupplied per mole of cobalt in the coating and unit time was 0.22l/min·mol.

Then, the suspension was subjected to solid-liquid separation using afilter press to collect the powder, and the powder was washed with waterand again subjected to filtration. Then, the obtained powder was driedin a vacuum drier at 120° C. for 20 hours to obtain 6.3 kg of a coatednickel hydroxide powder. The obtained coated nickel hydroxide powder wasdark brown. The powder was observed with a SEM to evaluate the state ofits coating. As a result, as shown in FIG. 1, the nickel hydroxideparticles were found to have a uniform coating layer.

The coated nickel hydroxide powder had, on the surface of particlesthereof, a coating mainly containing cobalt oxyhydroxide or a mixture ofcobalt oxyhydroxide and cobalt hydroxide, and the valence of cobalt inthe coating was 2.8. It is to be noted that the valence of cobalt in thecoating was determined in the following manner. First, trivalent cobaltwas analyzed, and the amount of bivalent cobalt was determined from thetotal amount of cobalt, and then the average valence of cobalt wascalculated. The analysis of trivalent cobalt was performed according toa method described in, for example, “Fractional determination of metalcobalt, cobalt(II), and cobalt(III) in cobalt oxide” (by Namiki Michikoand Hirokawa Kichinosuke, “BUNSEKI KAGAKU”, pp. 30 to 143, 1981), thatis, by a method in which a ferric chloride solution was used, andtitration with a potassium dichromate solution was performed usingsodium diphenylamine sulfonate as an indicator.

Then, 20 g of the coated nickel hydroxide powder was placed in apolyethylene jar (capacity: 50 ml, manufactured by SUNPLASTICS Co.,Ltd.) having a capacity of 50 ml. The polyethylene jar was tightlyclosed and shaken for 1 hour using a TURBULA shaker mixer (TURBULA TypeT2C manufactured by Willy A. Bachofen (WAB)). After the completion ofthe one-hour shaking, the presence or absence of peeling-off of thecobalt oxyhydroxide coating was determined. As a result, adhesion ofsmall pieces of the coating peeled off or the like to the inside of thejar was not observed, that is, peeling-off of the coating was notobserved.

After the completion of the shaking, 10 g of thecobalt-oxyhydroxide-coated nickel hydroxide powder was mixed with 200 mlof pure water with stirring, and the mixture was allowed to stand tosettle the cobalt-oxyhydroxide-coated nickel hydroxide powder toseparate the cobalt oxyhydroxide coating peeled off as a supernatant.The amount of the coating peeled off was determined by comparison ofcobalt content between the cobalt-oxyhydroxide-coated nickel hydroxidepowder after separation and the cobalt-oxyhydroxide-coated nickelhydroxide powder before shaking, and was found to be 14 mass % of thetotal mass of the coating.

It is to be noted that also when a substance other than the cobaltcompound such as cobalt oxyhydroxide or cobalt hydroxide is contained inthe coating, cobalt can be considered to be uniformly dispersed in thecoating; therefore, the amount of the coating peeled off can be measuredby the above measurement method.

Example 2

A reactor having a diameter of 84 cm and a depth of 100 cm was prepared,240 kg of the same spherical nickel hydroxide powder as used in Example1 having an average particle size of 8 μm was placed in the reactor, andwater was added to the reactor so that a total volume was 350 liters.Then, the nickel hydroxide powder was dispersed in the water by stirringusing a propeller stirrer at a rotation speed of 350 rpm to prepare asuspension of the nickel hydroxide powder.

Then, 80.7 liters of an aqueous cobalt sulfate solution adjusted to aconcentration of 1.6 mol/l was added in 2 hours using a roller pump from10 supply ports each having a diameter of 2 mm at an addition rate of67.2 ml/min per supply port to positions where the surface flow velocityof the suspension was 49.7 cm/sec. At the same time, a 24 mass % aqueoussodium hydroxide solution was added to its supply position under controlusing a roller pump interfacing with a pH controller so that the pH ofthe suspension was in the range of 10.2±0.2 as measured at 25° C. Thesupply position of the aqueous sodium hydroxide solution was 20 cm awayfrom the nearest one of contact portions between the aqueous cobaltsulfate solution supplied from the 10 supply ports and the surface ofthe suspension, and the surface flow velocity of the suspension in thesupply position of the aqueous sodium hydroxide solution was the same asdescribed above. In this way, cobalt hydroxide was deposited on thesurface of nickel hydroxide particles.

At this time, the ratio of the supply rate ρ (mol/sec) of the aqueouscobalt salt solution to the product of the supply width d (cm) of theaqueous cobalt salt solution in a direction orthogonal to the flowdirection of the suspension and the flow velocity v (cm/sec) of thesuspension in each of the contact portions between the surface of thesuspension and the aqueous cobalt salt solution supplied thereto, thatis, ρ/(d×v) was 1.80×10⁻⁴ mol/cm². Further, with regard to the supplyport for the aqueous cobalt salt solution nearest the supply position ofsodium hydroxide, the ratio of the distance D (cm) between the supplyposition of the aqueous cobalt salt solution and the supply position ofthe aqueous alkali solution to the ratio of the supply rate ρ of theaqueous cobalt salt solution to the product of the supply width d of theaqueous cobalt salt solution and the flow velocity v of the suspension,that is, D/{ρ/(d×v)} was 1.11×10⁵ cm³/mol. It is to be noted that thetemperature of the suspension during reaction was controlled to be 50°C.

As a result of the coating step described above, the total amount ofcobalt sulfate supplied to the suspension was deposited as cobalthydroxide on the surface of particles of the nickel hydroxide powder sothat a cobalt-hydroxide-coated nickel hydroxide powder whose particleshad a cobalt hydroxide coating on the surface thereof was obtained.

After the completion of the coating step, sodium hydroxide was furtheradded to the stirred suspension to increase the pH of the suspension to12.8. In this way, a slurry to be used in the oxidation step wasobtained. Then, air was blown into the slurry at a flow rate of 140l/min for 4 hours to oxidize cobalt hydroxide deposited on the surfaceof the nickel hydroxide particles in the coating step to cobaltoxyhydroxide. At this time, the total amount of oxygen supplied per moleof cobalt in the coating was 52.0 l/mol, and the amount of oxygensupplied per mole of cobalt in the coating and unit time was 0.22l/min·mol.

Then, the suspension was subjected to solid-liquid separation using afilter press to collect the powder, and the powder was washed with waterand again subjected to filtration. Then, the obtained powder was driedin a vacuum drier at 120° C. for 20 hours to obtain 252 kg of a coatednickel hydroxide powder. The obtained coated nickel hydroxide powder wasdark brown. The powder was observed with a SEM to evaluate the state ofits coating. As a result, the nickel hydroxide particles were found tohave a uniform coating as in the case of Example 1.

The coated nickel hydroxide powder had, on the surface of particlesthereof, a coating mainly containing cobalt oxyhydroxide or a mixture ofcobalt oxyhydroxide and cobalt hydroxide. The valence of cobalt in thecoating was determined in the same manner as in Example 1 and was foundto be 2.8. Further, the coated nickel hydroxide powder was shaken in thesame manner as in Example 1, and as a result, adhesion of small piecesof the coating peeled off or the like to the inside of the jar was notobserved, and the amount of the coating peeled off was 15 mass % of thetotal mass of the coating.

Example 3

A reactor having a diameter of 190 cm and a depth of 220 cm wasprepared, 2880 kg of the same spherical nickel hydroxide powder havingan average particle size of 8 μm as used in Example 1 was placed in thereactor, and water was added to the reactor so that a total volume was3000 liters. Then, the nickel hydroxide powder was dispersed in thewater by stirring using a propeller stirrer at a rotation speed of 150rpm to prepare a suspension of the nickel hydroxide powder.

Then, 968.3 liters of an aqueous cobalt sulfate solution adjusted to aconcentration of 1.6 mol/l was added in 2 hours using a roller pump from2 nozzles at an addition rate of 4035 ml/min per nozzle by spraying theaqueous cobalt sulfate solution in a circular pattern having a diameterof 500 mm onto the surface of the suspension where the surface flowvelocity of the suspension was 126.5 cm/sec. At the same time, a 24 mass% aqueous sodium hydroxide solution was added to its supply positionunder control using a roller pump interfacing with a pH controller sothat the pH of the suspension was in the range of 10.2±0.2 as measuredat 25° C. The supply position of the aqueous sodium hydroxide solutionwas 20 cm away from the nearest one of contact portions between theaqueous cobalt sulfate solution supplied from the two nozzles and thesurface of the suspension, and the surface flow velocity of thesuspension in the supply position of the aqueous sodium hydroxidesolution was the same as described above. In this way, cobalt hydroxidewas deposited on the surface of nickel hydroxide particles.

At this time, the ratio of the supply rate ρ (mol/sec) of the aqueouscobalt salt solution to the product of the supply width d (cm) of theaqueous cobalt salt solution in a direction orthogonal to the flowdirection of the suspension and the flow velocity v (cm/sec) of thesuspension in each of the contact portions between the surface of thesuspension and the aqueous cobalt salt solution supplied thereto, thatis, ρ/(d×v) was 1.70×10⁻⁵ mol/cm². With regard to the nozzle for theaqueous cobalt salt solution nearest the supply position of sodiumhydroxide, the ratio of the distance D (cm) between the supply positionof the aqueous cobalt salt solution and the supply position of theaqueous alkali solution to the ratio of the supply rate ρ of the aqueouscobalt salt solution to the product of the supply width d of the aqueouscobalt salt solution and the flow velocity v of the suspension, that is,D/{ρ/(d×v)} was 11.8×10⁵ cm³/mol. It is to be noted that the temperatureof the suspension during reaction was controlled to be 50° C.

As a result of the coating step described above, the total amount ofcobalt sulfate supplied to the suspension was deposited as cobalthydroxide on the surface of particles of the nickel hydroxide powder sothat a cobalt-hydroxide-coated nickel hydroxide powder whose particleshad a cobalt hydroxide coating on the surface thereof was obtained.

After the completion of the coating step, sodium hydroxide was furtheradded to the stirred suspension to increase the pH of the suspension to12.8. In this way, a slurry to be used in the oxidation step wasobtained. Then, air was blown into the slurry at a flow rate of 1680l/min for 4 hours to oxidize cobalt hydroxide deposited on the surfaceof the nickel hydroxide particles in the coating step to cobaltoxyhydroxide. At this time, the total amount of oxygen supplied per moleof cobalt in the coating was 52.0 l/mol, and the amount of oxygensupplied per mole of cobalt in the coating and unit time was 0.22l/min·mol.

Then, the suspension was subjected to solid-liquid separation using afilter press to collect the powder, and the powder was washed with waterand again subjected to filtration. Then, the obtained powder was driedin a vacuum drier at 120° C. for 20 hours to obtain 3165 kg of a coatednickel hydroxide powder. The obtained coated nickel hydroxide powder wasdark brown. The powder was observed with a SEM to evaluate the state ofits coating. As a result, the nickel hydroxide particles were found tohave a uniform coating as in the case of Example 1.

The coated nickel hydroxide powder had, on the surface of particlesthereof, a coating mainly containing cobalt oxyhydroxide or a mixture ofcobalt oxyhydroxide and cobalt hydroxide. The valence of cobalt in thecoating was determined in the same manner as in Example 1 and was foundto be 2.9. Further, the coated nickel hydroxide powder was shaken in thesame manner as in Example 1, and as a result, adhesion of small piecesof the coating peeled off or the like to the inside of the jar was notobserved, and the amount of the coating peeled off was 12 mass % of thetotal mass of the coating.

Example 4

A cobalt-hydroxide-coated nickel hydroxide powder was obtained in thesame manner as in Example 1 except that in the oxidation step, air wasblown into the slurry whose pH had been increased to 12.8 at a flow rateof 8.0 l/min for 2 hours to oxidize cobalt hydroxide deposited on thesurface of the nickel hydroxide particles in the coating step to cobaltoxyhydroxide. At this time, the total amount of oxygen supplied per moleof cobalt in the coating was 59.5 l/mol, and the amount of oxygensupplied per mole of cobalt in the coating and unit time was 0.50l/min·mol.

The obtained coated nickel hydroxide powder was dark brown. The powderwas observed with a SEM to evaluate the state of its coating. As aresult, the nickel hydroxide particles were found to have a uniformcoating as in the case of Example 1.

The coated nickel hydroxide powder had, on the surface of particlesthereof, a coating mainly containing cobalt oxyhydroxide or a mixture ofcobalt oxyhydroxide and cobalt hydroxide. The valence of cobalt in thecoating was determined in the same manner as in Example 1 and was foundto be 2.6. Further, the coated nickel hydroxide powder was shaken in thesame manner as in Example 1, and as a result, small dark brown adherentsvery slightly adhered to the inside of the jar, and the amount of thecoating peeled off was 20 mass % of the total mass of the coating.

Comparative Example 1

A cobalt hydroxide-coated nickel hydroxide powder was obtained in thesame manner as in Example 1 except that the rotation speed of thepropeller stirrer was changed to 300 rpm and that an aqueous cobaltsulfate solution and an aqueous sodium hydroxide solution were addedwhen the surface flow velocity of the suspension in a portion where theaqueous cobalt sulfate solution was to be added reached a steady stateof 5 cm/sec.

At this time, the ratio of the supply rate ρ of the aqueous cobalt saltsolution to the product of the supply width d of the aqueous cobalt saltsolution supplied to the suspension and the flow velocity v of thesuspension, that is, ρ/(d×v) was 4.48×10⁻⁴ mol/cm². Further, the ratioof the distance D between the supply position of the aqueous cobalt saltsolution and the supply position of the aqueous alkali solution to theratio of the supply rate ρ of the aqueous cobalt salt solution to theproduct of the supply width d of the aqueous cobalt salt solution andthe flow velocity v of the suspension, that is, D/{ρ/(d×v)} was0.335×10⁵ cm³/mol. It is to be noted that the temperature of thesuspension during reaction was controlled to be 50° C.

After the completion of the coating step, the pH of the suspension wasnot increased but was kept at 10.2±0.2, and the obtained suspension wasused as a slurry in the oxidation step. Air was blown into the slurryfor 4 hours in the same manner as in Example 1 to oxidize cobalthydroxide deposited on the surface of the nickel hydroxide particles inthe coating step to cobalt oxyhydroxide.

The obtained powder was washed, filtered, and dried in the same manneras in Example 1 to obtain a coated nickel hydroxide powder. The obtainedcoated nickel hydroxide powder was dark brown. The powder was observedwith a SEM to evaluate the state of its coating. As a result, as shownin FIG. 2, cobalt oxyhydroxide scales were observed in some positions onthe surface of the nickel hydroxide particles; therefore, the nickelhydroxide particles were found to be non-uniformly coated.

The coated nickel hydroxide powder had, on the surface of particlesthereof, a coating mainly containing cobalt oxyhydroxide or a mixture ofcobalt oxyhydroxide and cobalt hydroxide. The valence of cobalt in thecoating was determined in the same manner as in Example 1 and was foundto be 2.3. Further, the coated nickel hydroxide powder was shaken in thesame manner as in Example 1, and as a result, adhesion of small darkbrown particles to the inside of the jar was observed. The adheredparticles were analyzed by EDX, and as a result, cobalt was detected,from which it was confirmed that peeling-off of the coating occurred.Further, the amount of the coating peeled off was 29 mass % of the totalmass of the coating.

Comparative Example 2

A cobalt-oxyhydroxide-coated nickel hydroxide powder was obtained in thesame manner as in Example 2 except that the aqueous cobalt sulfatesolution was added using a roller pump in 2 hours at an addition rate of672.4 ml/min from one supply port having a diameter of 8 mm.

At this time, the ratio of the supply rate ρ of the aqueous cobalt saltsolution to the product of the supply width d of the aqueous cobalt saltsolution supplied to the suspension and the flow velocity v of thesuspension, that is, ρ/(d×v) was 4.51×10⁻⁴ mol/cm². Further, the ratioof the distance D between the supply position of the aqueous cobalt saltsolution and the supply position of the aqueous alkali solution to theratio of the supply rate ρ of the aqueous cobalt salt solution to theproduct of the supply width d of the aqueous cobalt salt solution andthe flow velocity v of the suspension, that is, D/{ρ/(d×v)} was0.443×10⁵ cm³/mol. It is to be noted that the temperature of thesuspension during reaction was controlled to be 50° C.

The obtained coated nickel hydroxide powder was dark brown. The powderwas observed with a SEM to evaluate the state of its coating. As aresult, cobalt oxyhydroxide scales were observed in some positions onthe surface of the nickel hydroxide particles as in the case ofComparative Example 1; therefore, the nickel hydroxide particles werefound to be non-uniformly coated.

The cobalt-oxyhydroxide-coated nickel hydroxide powder had, on thesurface of particles thereof, a coating mainly containing cobaltoxyhydroxide or a mixture of cobalt oxyhydroxide and cobalt hydroxide.The valence of cobalt in the coating was determined in the same manneras in Example 1 and was found to be 2.7. Further, the coated nickelhydroxide powder was shaken in the same manner as in Example 1, and as aresult, adhesion of small dark brown particles to the inside of the jarwas observed. The adhered particles were analyzed by EDX, and as aresult, cobalt was detected, from which it was confirmed thatpeeling-off of the coating occurred. Further, the amount of the coatingpeeled off was 26 mass % of the total mass of the coating.

Comparative Example 3

A coated nickel hydroxide powder was obtained in the same manner as inExample 1 except that after the completion of the coating step forcoating the surface of nickel hydroxide with cobalt hydroxide, the pH ofthe suspension was increased to 12.0 by adding sodium hydroxide to thestirred suspension to obtain a slurry to be used in the oxidation stepfor oxidizing cobalt hydroxide to cobalt oxyhydroxide and that air wasblown into the slurry at a flow rate of 3.5 l/min for 4 hours to oxidizecobalt hydroxide on the surface of the nickel hydroxide particles tocobalt oxyhydroxide. At this time, the total amount of oxygen suppliedper mole of cobalt in the coating was 52.0 l/mol, and the amount ofoxygen supplied per mole of cobalt in the coating and unit time was 0.22l/min·mol.

The obtained coated nickel hydroxide powder was slightly greenish darkbrown. The powder was observed with a SEM to evaluate the state of itscoating. As a result, the nickel hydroxide particles were found to havea uniform coating as in the case of Example 1.

The coated nickel hydroxide powder had, on the surface of particlesthereof, a coating mainly containing cobalt oxyhydroxide or a mixture ofcobalt oxyhydroxide and cobalt hydroxide. The valence of cobalt in thecoating was determined in the same manner as in Example 1 and was foundto be 2.4. Further, the coated nickel hydroxide powder was shaken in thesame manner as in Example 1, and as a result, adhesion of small darkbrown particles to the inside of the jar was observed. The adheredparticles were analyzed by EDX, and as a result, cobalt was detected,from which it was confirmed that peeling-off of the coating occurred.Further, the amount of the coating peeled off was 22 mass % of the totalmass of the coating.

As can be seen from the above Examples and Comparative Examples, thecoated nickel hydroxide powder of each of Examples has a uniform coatingon the surface of particles thereof, and the coating has high adhesionproperties and has been properly oxidized to cobalt oxyhydroxide.However, in Example 4 in which the total amount of oxygen supplied wassimilar to those of Examples 1 to 3 but the amount of oxygen suppliedper unit time was larger than those of Examples 1 to 3, there was atendency that the valence of cobalt was slightly lower, and the adhesionproperties of the coating was slightly reduced.

On the other hand, in Comparative Examples 1 and 2 in which the ratio ofthe supply rate (ρ) of the aqueous cobalt salt solution to the productof the supply width (d) of the aqueous cobalt salt solution and the flowvelocity (v) of the suspension in the coating step, that is, ρ/(d×v) waslarger than 3.5×10⁻⁴ mol/cm², the amount of the coating peeled off waslarger than those of Examples, and the adhesion properties of thecoating was inferior to that of Examples. Further, in ComparativeExample 3 in which the pH of the slurry in the oxidation step was lower,the valence of cobalt was lower than those of Examples, and the adhesionproperties of the coating was significantly reduced.

Further, the pressed powder resistance of each of the coated nickelhydroxide powders of Examples and Comparative Examples was measured. Asa result, the coated nickel hydroxide powders of Examples were found tohave higher conductivity than the coated nickel hydroxide powders ofComparative Examples. This shows that the coated nickel hydroxidepowders of Examples are suitable for a positive electrode activematerial of alkaline secondary battery.

INDUSTRIAL APPLICABILITY

The coated nickel hydroxide powder according to the present inventionhas high conductivity; therefore, its utilization ratio for a positiveelectrode active material of alkaline secondary battery is high.Therefore, the coated nickel hydroxide powder according to the presentinvention is suitable for use in a power source for portable electronicdevice required to have a high capacity. Further, the coated nickelhydroxide powder according to the present invention is suitable for usein a power source for electric car or hybrid car required to havehigh-output characteristics.

1-6. (canceled)
 7. A coated nickel hydroxide powder for a positiveelectrode active material of alkaline secondary battery comprisingnickel hydroxide powder particles whose surface is coated with a cobaltcompound mainly containing cobalt oxyhydroxide or a mixture of cobaltoxyhydroxide and cobalt hydroxide, wherein the coated nickel hydroxidepowder is produced by a method comprising: a coating step in which anaqueous cobalt salt solution and an aqueous alkali solution are suppliedto a stirred suspension obtained by dispersing a nickel hydroxide powderin water to form, on a surface of nickel hydroxide particles, a coatingmainly containing cobalt hydroxide crystallized out by neutralization;and an oxidation step in which oxygen is supplied to a stirred slurry ofthe nickel hydroxide particles having the coating formed thereon tooxidize cobalt hydroxide in the coating, wherein a valence of cobalt inthe coating is 2.5 or more and an amount of cobalt contained in thecoating is 90 mass % or more with respect to a total mass of metalelements contained in the coating, and wherein when 20 g of the coatednickel hydroxide powder is shaken in a closed container for 1 hour, anamount of the coating peeled off is 20 mass % or less of a total mass ofthe coating.
 8. The coated nickel hydroxide powder for a positiveelectrode active material of alkaline secondary battery according toclaim 7, wherein an upper limit of the valence of cobalt in the coatingis 3.